Power switches, circuit breakers, isolators and earth switches are central components in electrical switching systems and are maintained and checked at regular intervals to ensure they function correctly. In energy engineering plants, for example, power generation plants such as power stations or power transmission plants, circuit breakers switch not only operating currents, but also overload currents or short-circuit currents in the event of faults. Circuit breakers can be used both to switch on these currents and to switch off the currents. Preferably, the circuit breakers have extremely low contact resistance in a closed state in order to prevent unnecessary heating up of the circuit breaker and in the event of a residual current in which some ten kiloamps can flow, conduct safely and not overheat. Multi-pole circuit breakers are used to switch multi-phase currents. Each pole in the multi-pole circuit breaker switches one of the plurality of phases and comprises one or more switching contacts switched in series, so-called interrupters or interruption units. A switch in a three-phase network thus comprises three poles, each of which can again comprise one or more interrupters in series.
When testing a switch of this type, for example a circuit breaker, this is opened first and then separated from the rest of the system by means of separators and finally all the poles of the switch are earthed on both sides. There are two routine procedures when earthing a multi-pole switch, for example a multi-pole circuit breaker. It is common in large parts of Europe, for example, to directly connect each pole to earth individually on both sides by means of a corresponding grounding cable. In the USA, however, it is common to earth a first pole on both sides by means of corresponding grounding cables and then to connect the first pole to a second pole on both sides and then to connect the second pole to the third pole on both sides. This type of earth connection is also known as P-P-P-E. Regardless of the type of earth connection, it is advisable for safety reasons to always keep all poles of the switch earthed on both sides.
During the testing of an electrical circuit breaker, a so-called micro-ohm test is usually performed, in which, with the switching contact or interrupter of the electrical circuit breaker closed, the resistance occurring at the circuit breaker is measured by means of infeed of a predefined current and a voltage is measured by means of the switch, i.e. in the case of several interrupters per phase, by means of all interrupters combined. The predefined current can be several hundred amperes, for example 200 A. Furthermore, during the testing of the electrical circuit breaker, a so-called timing test can be performed, which registers a switching time that is required by the switch in order to execute the switching function command after a control command has been applied. However, if the circuit breaker is earthed on both sides, performing such checks is impractical since earthing on both sides influences the measurements.